1. Field of the Invention
The present invention relates to an optical control device, and more particularly, to an optical control device using an anisotropic dielectric substrate.
Priority is claimed on Japanese Patent Application No. 2009-088508, filed Mar. 31, 2009, the entire contents of which are incorporated herein by reference.
2. Description of Related Art
In the field of optical communications or optical measurement, optical control devices have become commercially available in which an optical waveguide, a signal electrode, and a ground electrode are formed on a substrate having an electro-optical effect, and a high-frequency signal is applied to the signal electrode to modulate an optical wave propagating through the optical waveguide.
For realization of high-speed bulk data transfers and various types of optical modulation, the shape of the optical waveguides used in the optical control device has become complicated. Thus, the number of signal electrodes for controlling the optical waveguides has tended to increase. For example, in modulation methods such as DQPSK (Differential Quadrature Phase Shift Keying) or SSB (Single Side Band) as disclosed in Patent Document 1, as shown in FIG. 1, a nested waveguide is used in which another Mach-Zehnder waveguides (sub Mach-Zehnder waveguides) 3 are incorporated into two branch waveguides 2 that constitute one Mach-Zehnder waveguide (main Mach-Zehnder waveguide) 1. Signal electrodes 4 are disposed so as to interact with the Mach-Zehnder waveguides 3. Moreover, for realization of further higher-speed modulation, other modulation methods have been proposed such as polarization multiplexing in which the structures for DQPSK are arranged in parallel to each other so as to perform respective modulation with orthogonal polarization waves, or D8PSK or D16PSK that further divides phase information.
Moreover, optical control devices in which signal path switching is achieved in an optical waveguide have been studied. Examples of such optical control devices include an optical control device that uses a directional coupler as disclosed in Patent Document 2, an optical control device as disclosed in Patent Document 3, in which arms have a large refractive index difference so that light is concentrated on an arm having a high refractive index, an optical control device that uses a total reflection as disclosed in Patent Document 4, and an optical control device that uses a Mach-Zehnder interferometer as disclosed in Patent Document 5.
However, when there is a plurality of signal electrodes, it is necessary that electrical signals applied to the respective signal electrodes arrive at an acting portion (between the points b and c in FIG. 1) S where the electrical signals interact with the optical waves propagating through the optical waveguides at a predetermined time. For this reason, it is essential to consider the propagation time of the high-frequency signal through an input-side signal electrode portion I in each signal electrode ranging from a signal input terminal portion 5 (point a) formed by an electrode pad to a starting point (point b) of the acting portion S.
An example of a method of considering the propagation time of the electrical signal in the input-side signal electrode portion I includes a method of adjusting the timing of inputting the electrical signal to the signal input terminal portion 5 considering the length of the input-side signal electrode portion I of each signal electrode and a method of adjusting the lengths of the input-side signal electrode portions I of the signal electrodes so as to be the same as disclosed in Patent Document 6 or 7.
On the other hand, when material (anisotropic dielectric substrate) having anisotropy in dielectric constant is used, since the high-frequency refractive index also has anisotropy, the propagation speed, impedance and propagation loss of the electrical signals may change depending on propagation direction. For this reason, modulation electrodes may be formed on a substrate having such an electro-optical effect that the dielectric constant is different in each orthogonal direction parallel to the substrate surface. In this case, for example, even when the effective electrode lengths of the input-side signal electrode portions I of the respective signal electrodes are adjusted to be the same, since the electrical reflection and propagation loss resulting from an impedance mismatch is different in each modulation electrode, the modulation characteristics will be deteriorated. In addition, since the refractive index of the electrical signal changes depending on the frequency of the electrical signal, there is a problem in that when electrical signals having different frequencies are input to modulation electrodes having the same shape, the modulation start time is different in each modulation electrode.